0-> IMAGE_ROOTFS_ALIGNMENT reserved to bootloader (not partitioned)
IMAGE_ROOTFS_ALIGNMENT -> BOOT_SPACE kernel and other data
BOOT_SPACE -> SDIMG_SIZE rootfs
Use IMAGE_OVERHEAD_FACTOR to add more space
Please, go to original file in order to understand the disk layout.
It´s basically some initial space for u-boot. One partition for uImage. One partition for rootfs. The total sdcard size will be calculate for every image, if you want to add more empty space inside generated sdcard, use IMAGE_OVERHEAD_FACTOR.